The problem is that when you add a user mention to a post, then it's going to post that to their Facebook timeline (telling them that you mentioned them on the post). If you later come back and remove them, then it really needs to remove the mention from their timeline as well, because you're no longer mentioning them.

Updating mentions on timelines like that turns out to be very tricky, so for the moment, you only can mention people/pages when adding new posts, not when editing old ones. Like Matt said, this may be added in the future.

Deleting the mentions en-masse when a post is removed was much easier and works fine.